We had a problem when running our last keyserver migraiton on Ashoat's keyserver where the migration took much longer than prior migrations. The AWS load balancer's default grace period for the ecs task ran out and
the migration process was cut off in the middle. This diff runs a temporary health check express server during the migration process so AWS can establish the ecs task as healthy
Was originally worried that we would need to fully stop the health check express server, but given that it's only on the master process, I don't believe this is necessary. Additionally, the default `close` provided function prevents new connections